Ideally in limit poker, you want to sit down at each table with around 25BB and have a bankroll of 300-500 BB. The more is better. If you're trying to play 2/4 with a $500 bankroll, one bad run could cripple you. Even $500 is a little small for 1/2, but it's not horrible. I'd get some 1/2 action in first, and if you can show a winning rate, move up to 2/4 when you get to about $1200 or even more. I do not know much about NL bankrolls. You may want to post there to see what they have to say.